This invention relates to audio amplifiers combining thermionic valves and solid state amplification devices.
Audio amplifiers employing thermionic valves, most commonly known as vacuum tubes, (hereinafter "tube amplifiers") remain highly prized by audiophiles for their sonic characteristics. Such characteristics include a "warmth" and "coloring" of musical sound that, it is generally believed, cannot be provided by audio amplifiers employing only solid state amplification devices.
Some particular tube amplifiers are sought for additional reasons. The sonic characteristics of tube guitar amplifiers is desired by more than a generation of young people who have grown up associating such characteristics with rock and roll bands since the 1950's. Particularly, amplifiers manufactured by Fender Musical Instruments Corp. of Scottsdale, Ariz. and marketed under the trademark FENDER (hereinafter "Fender") and amplifiers manufactured by Marshall Amplification of Bletchley, Milton Keynes, UK under the trademark MARSHALL (hereinafter "Marshall"), have become through now hallowed tradition the rock and roll amplifiers of choice. Musicians and their fans have become accustomed to the sound produced by these types of amplifiers to such an extent that their characteristics are inseparable from the characteristics of the guitar. These characteristics are all the more identifying because guitar amplifiers are commonly driven into non-linear operation where large signal distortion characteristics become pronounced. Consequently, solid state guitar amplifiers in particular have not been successful in faithfully reproducing the characteristic sounds of earlier vacuum tube designs, despite their superior energy efficiency, lighter weight and greater reliability.
In practice, performers will either switch between a Fender amplifier and its associated speaker and a Marshall amplifier and its associated speaker during a performance to produce different sounds, or use a guitar amplifier which employs channel switching techniques to perform the same function, such as exemplified in Smith, U.S. Pat. No. 4,701,957, and Brown, Sr. et al., U.S. Pat. Nos. 5,023,915 and 4,890,331. In the former case, either a foot switch or stereo/pan pedal is used to change the output of the guitar from the input of one amplifier to the input of the other. In the case of channel switching, a foot switch is used to change the circuit characteristics, e.g., topology, of the amplifier. The need to provide and employ two guitar amplifiers in the former case is a problem, and in either case the amplifiers have all of the typical disadvantages of tube amplifiers. That is, tube amplifiers have relatively poor reliability, and are heavy, bulky and expensive in comparison to solid state amplifiers.
There have been efforts to combine the use of transistors with thermionic valves in amplifier design. For example, Robinson, U.S. Pat. No. 5,148,116 ("Robinson"), proposes providing a feed-back voltage from a transistor output stage to the control grid of a vacuum tube. Butler, U.S. Pat. No. 5,705,950 ("Butler"), employs a solid state amplifier stage followed by a vacuum tube stage. The basic amplifier in Butler may be incorporated as a predriver circuit into a multistage amplifier.
Such attempts have been successful to introduce some of the characteristics of tube sound in an audio power amplifier employing solid state amplification. However, the sonic characteristics of an amplifier include both the small and large signal distortion. In prior art attempts at emulating the characteristics of tube amplifiers in solid state amplifiers, the architectures employed have generally not retained the tube amplifier's large signal distortion characteristics. For example, both Robinson and Butler propose to employ vacuum tube amplifier stages essentially as preamplifiers which are not intended to operate in large signal distortion mode. And where vacuum tubes are employed in the output stage of a hybrid amplifier adapted to provide sufficient power to drive a loudspeaker, many of the disadvantages of tube amplifiers are retained.
Accordingly, there is a need for a hybrid thermionic valve and solid state audio amplifier that provides for the sonic characteristics of a vacuum tube amplifier along with the desirable characteristics of a solid state amplifier, such as greater efficiency and reliability, and lower cost and weight.